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[quake]$ ln -s /tmp/foo /etc/passwd
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Now if we cat out /tmp/foo, we get a listing of the password file This seemingly benign feature is a root compromise waiting to happen Although it is most common to abuse scratch files that are created in /tmp, there are applications that create scratch files elsewhere on the file system Let s examine a real-life symbolic-link vulnerability to see what happens In our example, we are going to study the dtappgather exploit for Solaris Dtappgather is a utility shipped with the common desktop environment Each time dtappgather is executed, it creates a temporary file named /var/dt/appconfig/ appmanager/generic-display-0 and sets the file permissions to 0666 It also changes the ownership of the file to the UID of the user who executed the program Unfortunately, dtappgather does not perform any sanity checking to determine if the file exists or if it is a symbolic link Thus, if attackers were to create a symbolic link from /var/dt/appconfig/appmanager/generic-display-0 to another file on the file system (for example, /etc/passwd), the permissions of this file would be changed to 0666 and the ownership of the file would change to that of the attackers We can see before we run the exploit, the owner and group permissions of the file /etc/passwd are root:sys
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[quake]$ ls -l /etc/passwd -r-xr-xr-x 1 root sys 560 May 5 22:36 /etc/passwd
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Next, we will create a symbolic link from named /var/dt/appconfig/ appmanager/ generic-display-0 to /etc/passwd
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[quake]$ ln -s /etc/passwd /var/dt/appconfig/appmanager/generic-display-0
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Finally, we will execute dtappgather and check the permissions of the /etc/passwd file
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[quake]$ /usr/dt/bin/dtappgather MakeDirectory: /var/dt/appconfig/appmanager/generic-display-0: File exists [quake]$ ls -l /etc/passwd -r-xr-xr-x 1 gk staff 560 May 5 22:36 /etc/passwd
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U Symlink Countermeasurebest countermeasure available Unfortunately, many proSecure coding practices are the
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Dtappgather blindly followed our symbolic link to /etc/passwd and changed the ownership of the file to our user ID It is also necessary to repeat the process on /etc/shadow Once the ownership of /etc/passwd and /etc/shadow are changed to our user ID, we can modify both files and add a 0 UID (root equivalent) account to the password file Game over in less than a minute s work
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grams are coded without performing sanity checks on existing files Programmers should check to see if a file exists before trying to create one, by using the O_EXCL | O_CREAT flags When creating temporary files, set the UMASK and then use tmpfile() or mktemp() functions If you are really curious to see a small complement of programs that create temporary files, execute the following in /bin or /usr/sbin/
[quake]$ strings * |grep tmp
8:
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If the program is SUID, there is a potential for attackers to execute a symlink attack As always, remove the SUID bit from as many files as possible to mitigate the risks of symlink vulnerabilities Finally, consider using a tool like L0pht Watch that monitors /tmp activity and informs you of programs that create temporary files L0pht Watch can be obtained from http://wwwL0phtcom/advisories/l0pht-watchtargz
File Descriptor Attacks
Popularity: Simplicity: Impact: Risk Rating: 2 6 9 6
File descriptors are nonnegative integers that the system uses to keep track of files rather than using specific filenames By convention, file descriptors 0, 1, and 2 have implied uses that equate to standard input, standard output, and standard error, respectively Thus, when the kernel opens an existing file or creates a new file, it returns a specific file descriptor that a program can use to read or write to that file If a file descriptor is opened read/write (O_RDWR) by a privileged process, it may be possible for attackers to write to the file while it is being modified Therefore, attackers may be able to modify a critical system file and gain root access Oddly enough, the ever-bulletproof OpenBSD was vulnerable to a file descriptor allocation attack in version 23 Oliver Friedrichs discovered that the chpass command used to modify some of the information stored in the password file did not allocate file descriptors correctly When chpass was executed, a temporary file was created that users were allowed to modify with the editor of their choice Any changes were merged back into the password database when the users closed their editor Unfortunately, if attackers shelled out of the editor, a child process was spawned that had read/write access to its parent s file descriptors The attackers modified the temporary file (/tmp/ptmp) used by chpass by adding a 0 UID account with no password When the attackers closed the editor, the new account was merged into /etc/masterpasswd and root access was granted Let s look at exactly how this vulnerability is exploited First, we change our default editor to vi because it allows a user to execute a shell while it is running:
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